netdev - [PATCH v3 3/5] netlink: allow sending extended ACK with cookie on success

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Date:   Sat,  8 Apr 2017 22:24:32 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     linux-wireless@...r.kernel.org, netdev@...r.kernel.org
Cc:     pablo@...filter.org, Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Benc <jbenc@...hat.com>,
        David Ahern <dsa@...ulusnetworks.com>, jiri@...nulli.us,
        Johannes Berg <johannes.berg@...el.com>
Subject: [PATCH v3 3/5] netlink: allow sending extended ACK with cookie on success

From: Johannes Berg <johannes.berg@...el.com>

Now that we have extended error reporting and a new message format
for netlink ACK messages, also extend this to be able to return
arbitrary cookie data on success.

This will allow, for example, nl80211 to not send an extra message
for cookies identifying newly created objects, but return those
directly in the ACK message.

The cookie data size is currently limited to 20 bytes (since Jamal
talked about using SHA1 for identifiers.)

Thanks to Jamal Hadi Salim for bringing up this idea during the
discussions.

Signed-off-by: Johannes Berg <johannes.berg@...el.com>
---
 include/linux/netlink.h      |  7 +++++++
 include/uapi/linux/netlink.h |  4 ++++
 net/netlink/af_netlink.c     | 38 ++++++++++++++++++++++++--------------
 3 files changed, 35 insertions(+), 14 deletions(-)

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 746359c27396..4bf13bb725ea 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -62,17 +62,24 @@ netlink_kernel_create(struct net *net, int unit, struct netlink_kernel_cfg *cfg)
 	return __netlink_kernel_create(net, unit, THIS_MODULE, cfg);
 }
 
+/* this can be increased when necessary - don't expose to userland */
+#define NETLINK_MAX_COOKIE_LEN	20
+
 /**
  * struct netlink_ext_ack - netlink extended ACK report struct
  * @_msg: message string to report - don't access directly, use
  *	%NL_SET_ERR_MSG
  * @bad_attr: attribute with error
  * @missing_attr: number of missing attr (or 0)
+ * @cookie: cookie data to return to userspace (for success)
+ * @cookie_len: actual cookie data length
  */
 struct netlink_ext_ack {
 	const char *_msg;
 	const struct nlattr *bad_attr;
 	u16 missing_attr;
+	u8 cookie[NETLINK_MAX_COOKIE_LEN];
+	u8 cookie_len;
 };
 
 /* Always use this macro, this allows later putting the
diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index 5f1a48b140c8..26625355ac97 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -118,6 +118,9 @@ struct nlmsgerr {
  * @NLMSGERR_ATTR_OFFS: error offset in the original message (u32)
  * @NLMSGERR_ATTR_ATTR: top-level attribute that caused the error
  *	(or is missing, u16)
+ * @NLMSGERR_ATTR_COOKIE: arbitrary subsystem specific cookie to
+ *	be used - in the success case - to identify a created
+ *	object or operation or similar (binary)
  * @NUM_NLMSGERR_ATTRS: number of attributes
  * @NLMSGERR_ATTR_MAX: highest attribute number
  */
@@ -126,6 +129,7 @@ enum nlmsgerr_attrs {
 	NLMSGERR_ATTR_MSG,
 	NLMSGERR_ATTR_OFFS,
 	NLMSGERR_ATTR_ATTR,
+	NLMSGERR_ATTR_COOKIE,
 
 	NUM_NLMSGERR_ATTRS,
 	NLMSGERR_ATTR_MAX = NUM_NLMSGERR_ATTRS - 1
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index c74f56a4fcf1..78f33b8011d2 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2311,6 +2311,9 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 			if (extack->missing_attr)
 				acksize += nla_total_size(sizeof(u16));
 		}
+	} else if (nlk->flags & NETLINK_F_EXT_ACK) {
+		if (extack && extack->cookie_len)
+			acksize += nla_total_size(extack->cookie_len);
 	}
 
 	skb = nlmsg_new(acksize, GFP_KERNEL);
@@ -2336,20 +2339,27 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 	       !(nlk->flags & NETLINK_F_CAP_ACK) ? nlh->nlmsg_len
 						 : sizeof(*nlh));
 
-	if (err && nlk->flags & NETLINK_F_EXT_ACK && extack) {
-		if (extack->_msg)
-			WARN_ON(nla_put_string(skb, NLMSGERR_ATTR_MSG,
-					       extack->_msg));
-		if (extack->bad_attr &&
-		    !WARN_ON((u8 *)extack->bad_attr < in_skb->data ||
-			     (u8 *)extack->bad_attr >= in_skb->data +
-						       in_skb->len))
-			WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_OFFS,
-					    (u8 *)extack->bad_attr -
-					    in_skb->data));
-		if (extack->missing_attr)
-			WARN_ON(nla_put_u16(skb, NLMSGERR_ATTR_ATTR,
-					    extack->missing_attr));
+	if (nlk->flags & NETLINK_F_EXT_ACK && extack) {
+		if (err) {
+			if (extack->_msg)
+				WARN_ON(nla_put_string(skb, NLMSGERR_ATTR_MSG,
+						       extack->_msg));
+			if (extack->bad_attr &&
+			    !WARN_ON((u8 *)extack->bad_attr < in_skb->data ||
+				     (u8 *)extack->bad_attr >= in_skb->data +
+							       in_skb->len))
+				WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_OFFS,
+						    (u8 *)extack->bad_attr -
+						    in_skb->data));
+			if (extack->missing_attr)
+				WARN_ON(nla_put_u16(skb, NLMSGERR_ATTR_ATTR,
+						    extack->missing_attr));
+		} else {
+			if (extack->cookie_len)
+				WARN_ON(nla_put(skb, NLMSGERR_ATTR_COOKIE,
+						extack->cookie_len,
+						extack->cookie));
+		}
 	}
 
 	netlink_unicast(in_skb->sk, skb, NETLINK_CB(in_skb).portid, MSG_DONTWAIT);
-- 
2.11.0